The present invention relates to implantable medical devices and to covered implantable stents comprising an elastic or superelastic material.
Stents are support structures that are implanted in tubular organs, blood vessels or other tubular body lumens to help keep such conduits open. Stents are often used following balloon angioplasty to prevent restenosis and may, more generally, be used in repairing any of a number of other tubular body lumens, such as those in the vascular, biliary, genitourinary, gastrointestinal, respiratory and other systems.
The materials used in fabricating stents must be chemically and biologically inert to living tissue. Stents must further be able to stay in position and continue to support the tubular body lumens into which they are implanted over extended periods of time. Moreover, stents must have the ability to expand from a contracted state, which facilitates insertion into a body lumen, to an expanded diameter that is useful in supporting at least a portion of the body lumen. This expansion is accomplished either mechanically, such as by the action of a balloon-ended catheter, or by self-expansion such as by shape-memory effects or by the use of a constrained elastic stent.
The above requirements limit the number of eligible stent materials. One of the most widely used metal alloy systems is the nickel-titanium system, the alloys of which are known as nitinol. Under certain conditions, nitinol is highly elastic such that it is able to undergo extensive deformation and yet return to its original shape. Elastic stents are typically deployed in a body lumen by reducing the diameter of the stent by mechanical means, restraining the stent in the reduced diameter during insertion into the body, and releasing the stent from the restraint at a target location. Once released, the stent xe2x80x9cself-expandsxe2x80x9d to its predetermined, useful diameter by virtue of its elastic properties. One of the advantages of elastic stents is that, after deployment, they are able to xe2x80x9creboundxe2x80x9d to their useful diameters after being deformed by external forces. The resilient nature of such stents not only make them ideal for self-expansion upon delivery to a target location, but it also makes them desirable for use in body lumens that are often subjected to external forces and corresponding temporary reductions in diameter or other deformations. For example, elastic stents are useful for placement in the carotid artery, which is frequently deformed by external forces because the vessel is in close proximity to the body surface.
There are, however, some potential drawbacks associated with conventional elastic stents. For example, such self-expanding stents possess a single predetermined diameter, thus limiting the use of a given stent and increasing the number of different stents required to cover a range of useful diameters. Where the predetermined diameter is larger than the body lumen in which the stent is placed, residual expansion forces often result in the gradual, undesired expansion of the surrounding lumen. The release of self-expanding stents often does not exert sufficient force to open blocked body lumens containing hard plaque. It is thus sometimes necessary to perform the additional step of inserting a balloon into the partially-deployed stent for further dilation, thus adding cost, time and risk to the overall procedure.
The present invention provides stents for deploying within tubular organs, blood vessels, or other tubular body lumens. Such stents comprise a stent body comprising an elastic material, the stent body being characterized by a free cylindrical shape having a free diameter. The stent body is at least partially covered with a covering that substantially prevents the stent body from expanding towards its free diameter when the stent body is placed into a diameter smaller than the free diameter. In addition, after the stent is expanded, the covering does not force the stent body to a diameter smaller than the diameter to which it is expanded. In one embodiment, the covering is a metal coating on the stent body. In another embodiment, the covering is a tube around the stent body. In yet another embodiment, the covering includes multiple rings around the stent body.
The present invention also provides methods for deploying the stents of the present invention within tubular organs, blood vessels, or other tubular body lumens. The method includes the steps of providing a stent comprising a stent body comprising an elastic material, the stent body being characterized by a free cylindrical shape having a free diameter; deforming the stent to a diameter smaller than the free diameter; covering the stent with a covering that substantially prevents the stent body from expanding towards the free diameter when the stent body is placed into a diameter smaller than the free diameter; inserting the stent into the body while in the reduced diameter; and mechanically expanding the stent. In one embodiment, the step of covering the stent comprises the step of coating the stent body. In another embodiment, the step of covering the stent comprises the step of placing a tube over the stent. In yet another embodiment, the step of covering the stent comprises the step of placing multiple rings over the stent.
One advantage of the present invention is that it provides stents that are isothermally deployed in the body.
Another advantage of the present invention is that it provides stents that resist loads exerted by a surrounding tubular organ.
Another advantage of the present invention is that it provides expanded stents that are reboundable and resilient along their longitudinal and radial axes, and thus resist deformation when exposed to longitudinal and radial forces.
Yet another advantage of the present invention is that it provides stents that are expanded to controlled, desired dimensions by balloon catheters, and each stent can be expanded to any diameter within a wide range of diameters.